1. Field of the Art
The present invention relates to a fluidized bed boiler, and more particularly to a fluidized bed water tube boiler in which at least a fluidized bed wall is constituted of water tubes.
2. Prior Art
A fluidized bed water tube boiler comprises a fluidized bed combustion section, a freeboard section and a convective heat transfer section. First, a conventional fluidized bed water tube boiler will be described with reference to FIG. 8.
In FIG. 8, a fluidized bed water tube boiler comprises a fluidized bed combustion section 1 and a freeboard section 2 which are formed from a common water tube wall 11. Combustion gas generated in the fluidized bed combustion section 1 passes through the freeboard section 2 and is then introduced to a convective heat transfer section 3 provided in the boiler, and is then discharged to the outside through an exhaust gas outlet 10 after being subjected to heat exchange.
Boiler water is led out of a water drum 5, rises in the common water tube wall 11, and further enters a steam drum 4. In the convective heat transfer section 3, the boiler water moves down through tubes in a rear wall 13 of the convective heat transfer section to enter the water drum 5 and, thereafter, moves up through heat transfer tubes 14 in the convective heat transfer section while being heated, and returns to the steam drum 4. Thus, the fluidized bed water tube boiler is constituted such that the steam drum, the water drum and the water tubes cooperate to circulate the boiler water. The generated steam is taken out through a steam outlet 27.
In the operation of the fluidized bed water tube boiler shown in FIG. 8, fluidizing air is introduced into an air plenum 7 through a fluidizing air inlet 6. Then, the air is blown at 3-12 Uo/Umf into the fluidized bed combustion section through fluidizing air dispersion nozzles 9 provided at the furnace bottom, so that a fluidizing medium in the fluidized bed combustion section is fluidized. Fuel is supplied through a fuel supply port 28 in the heated fluidizing medium.
While the fuel is burnt in the fluidized bed combustion section, fuel powder not yet burnt and exiting the fluidized bed, as well as combustible volatile components produced by being heated in the fluidized bed are further burnt with secondary air supplied to the freeboard section. Exhaust gas is discharged from an upper portion of the freeboard section in the direction of an arrow (.fwdarw.) and is then introduced to the convective heat transfer section 3 from its upper portion, and is discharged to the outside through the exhaust gas outlet 10 after heating the boiler water in the convective heat transfer section 14.
Incidentally, in FIG. 8, reference numeral 8 denotes a refractory material for protecting the inner combustion surface of the fluidized bed and 15' denotes a heat transfer tube in the fluidized bed formed by bending the water tube.
Meanwhile, there are known various types of fluidized bed water tube boilers wherein the fluidized bed is of a swirling type, or wherein an inclined partition wall is provided in the swirling fluidized bed to define a heat recovery chamber between a rear surface of the inclined partition wall and a furnace wall.
Since a combustion rate and other factors differ depending on the kinds of fuel and materials to be burnt, boilers are generally designed to be adapted for the kind of fuel and combustibles used. In particular, since a freeboard has an important role, its volume and configuration are made to different specifications depending on the use.
Accordingly, it is difficult to change the fuel once a furnace is in operation. In the case of burning industrial waste, there is a problem that the properties of exhaust gas deteriorate with changes in the nature of waste to be burnt.
Fluidized bed boilers are designed to be adaptable for various kinds of fuel, and can be used to burn a wide range of materials such as low-grade coal and industrial waste. However, such an advantage of fluidized bed boilers has not yet been fully utilized for the reason mentioned above.
Further, since designs depend on the fuel used, it has been difficult to produce a standardized design even for medium- and small-sized fluidized bed boilers.
The foregoing problems are basically attributable to the nature of the water tube boiler itself. More specifically, water tube boilers are superior both in terms of structure and function, and most boilers used today for industrial or utility purposes are water tube boilers.
With respect to the design of water tube boilers, it is important to ensure proper circulation of boiler water through the water tube wall and the heat transfer tubes. Thus, the boiler's configuration and structure are determined with a view thereto, and to the nature of a fuel to be used and the level of combustion which is expected.
Accordingly, if it is attempted to change fuel after a furnace is in use, a deterioration in combustion and in the properties of an exhaust gas generally result. It is difficult and costly to change the freeboard structure of a boiler to solve such a problem, since the water tubes themselves are an integral part of the boiler and function as a pressure part. Furthermore, water circulation must be maintained at an adequate level after any such reconstruction.
Although a fluidized bed water tube boiler is often utilized to burn solid fuel such as coal, it has a smaller furnace load than heavy oil boilers or gas-fueled boilers, and requires a lower flow velocity in the convective heat transfer section to overcome the problem of dust in exhaust gas. This necessarily leads to an increase in size of the fluidized bed combustion section, the freeboard section and the convective heat transfer section. Thus, even a boiler with a steam level of 10-20 T/H, it must be assembled on site due to the transport limitations.
On the other hand, heavy oil boilers or gas-fueled boilers even in the class of 10-20 T/H can be manufactured in a factory and transported as a completed product to a site with no need for further assemblance. Accordingly, those boilers are superior to fluidized bed water tube boilers from standpoints of cost and construction efficiency. Consequently, fluidized bed water tube boilers have not generally been used for burning coal.